1. Field of the Invention
A method and apparatus for minimizing multipath interference for commercial FM signals, which use their subsidiary communications authority (SCA) to transmit secondary signals in a quadrature amplitude modulation (QAM) format to reduce or eliminate interference in QAM signals caused by phase abnormalities present in every day FM signals.
2. Description of Related Art
In the United States, FM radio stations are allocated 200 KHz over which to broadcast their signal, which signal actually only takes up about 76 KHz in the stereo format. Under the defining Federal regulations, FM stations have been able to use the excess bandwidth to transmit additional program material on a subscription basis to produce additional revenue. The secondary signals for this subsidiary communications authority, SCA, are modulated onto the station carrier frequency above the usual 38 KHz base band signal.
Various modulation methods have been used for SCA signals and others have been suggested. Quadrature amplitude modulation (QAM) has been suggested and tested because of its capacity to carry sufficient data to provide a quality music signal. However, this data capacity is available because signal phase is used to encode the data, making QAM signals dependent upon signal phase for accurate data demodulation. Whereas the normal analog FM broadcast is not adversely affected by phase and phase is not used in its demodulation, QAM and other similar modulation methods are severely restricted because of phase abnormalities present in everyday FM signals. Phase abnormalities caused by the Doppler effect are an inherent part of relative movement between transmitter and receiver and thus restrict practical use of such SCA QAM signals for vehicular applications. Also troublesome, are the phase abnormalities caused by common multipath interference. Multipath interference causes step changes in the phase of the received signal because the actual signal being received is changing between a direct signal from the transmitter and one or more reflected versions of the direct signal. The reflected signals have a different phase because they have taken a longer path to the receiver. Thus when such a change occurs in the received signal, a QAM demodulator looses its phase reference for demodulation. Although a receiver can recover, over time, from such a change, normal vehicle environments can cause many such changes to occur making a QAM SCA signal unusable.
A method and apparatus for minimizing multipath interference for commercial FM signals, which use their SCA (subsidiary communications authority) to transmit secondary signals in a QAM (quadrature amplitude modulation) format, tracks a time series of phase measurements of the received 19 KHz stereo pilot signal to determine the proper phase reference for demodulation of the QAM signal at approximately the center of the time series. An algorithm compares the phase measurements over the time series to detect sudden changes, as caused by multipath signal interference, as well as to detect gradual changes in phase as caused by relative movement between transmitter and receiver. The appropriate phase reference is then chosen for demodulation purposes. One such algorithm is implemented by a median filter, which reorders measured phase values by amplitude and chooses the value at the center of the reordered series. Operation of the algorithm is aided by removing the normal 2Π per cycle phase rotation of the pilot signal as part of the phase measurement.
It is an object of this invention to minimize multipath interference for commercial FM signals that use SCA to transmit secondary signals in a QAM quadrature amplitude modulation format by eliminating phase abnormalities that cause interference present in every day FM signals.
It is another object of this invention to provide a method and apparatus for minimizing multipath interference in the receiver from wireless commercial transmissions that utilize commercial FM signals having subsidiary communications authority to transit secondary signals in a QAM format by tracking the phase measurement of the pilot signal to determine proper phase reference for demodulation of the QAM signal.
In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.